In response to the world wide need of modern societies for the indication and control of the humidity of myriad processes and locations in commerce, industry, the sciences, and the consumer world, scores of hygrometric devices have been developed. Nearly all of these have been "secondary" devices which depend on non-reproducible processes (such as moisture sorption by various materials) and as a result the devices have no inherent accuracy, drift badly with time, and are only suitable for undemanding needs.
Only a very few inherently accurate "primary" electronic devices based on unvarying physical phenomena have been developed. These have been characterized by very high cost, high power consumption, and large sensor bulk and equipment size. Thus, myriad applications are being served inadequately at present. Heading the very short list of primary electronic instruments is the "dew temperature" or "cold mirror" hygrometer, shown in its modern, thermoelectrically cooled version in FIG. 1. Its flexibility of application, ease of use, and accuracy make it the primary instrument of choice for the majority of demanding applications where its high cost, bulk, and power needs can somehow be tolerated. A primary device free of these limitations has been badly needed. Such a device would permit significant improvements in control and product quality in many applications of importance.
A study of my U.S. Pat. Nos. 3,776,038, 3,863,502, 4,166,891, and 4,175,207 suggests that a new type of primary hygrometer might be possible, based on the teachings of these patents. It would be analogous to the cold mirror hygrometer of FIG. 1. However, instead of securing an optical signal from dew formation at a particular temperature on a cooled mirror, a phase shift phenomenon, the optical signal would come from the phase shift at a particular temperature of a single suitable salt on a heated mirror. The phase shift could be solid/solid (an isotropic crystal changing to an anisotropic crystal) or solid/liquid (an anisotropic solid changing to an isotropic solution). There is a wider choice of sensing salts which undergo the solid/liquid phase shift than of those which undergo the solid/solid change. Thus, the solid/liquid change would be the sensing mode of greatest interest. However, the same concepts would apply to the solid/solid mode of sensing.
If the art taught in the patents mentioned were used, and especially if polarized light were used for signal amplification, a suitable humidity sensing system would then include these basics: (1) a source of illumination which provides a light beam, (2) means for providing observable changes in humidity comprising a chemical composition which of itself can sense changes in humidity, the chemical composition (through a change in the degree of hydration) being abruptly triggered to an isotropic/ anisotropic phase change at a particular water vapor pressure/temperature, (3) means for heating said composition to the phase change temperature of the particular chemical composition for that water vapor pressure, (4) means for determining the temperature of the chemical composition at the phase equilibrium point, (5) light detecting means for detecting a pre-selected brightness of light emerging from the film at the equilibrium point and so providing a signal, and (6) an empirically established graph of water vapor pressure vs temperature defining the particular chemical composition's equilibrium curve for the phases sensed. From this curve the dew temperature equivalent of any temperature on the equilibrium curve can be readily determined.
Such a device would have the same useful characteristic as dew temperature hygrometers, namely, giving a continuous readout of humidity, e.g. from 5.0% RH to 100% RH, from a single salt. By way of comparison, the direct readout hygrometers now being made commercially under these patents also offer high utility and stability, and freedom from drift. However, they give discrete readouts of humidity, e.g. 30.0% RH, 32.5% RH, 35.0% RH, 37.5% RH, etc. A continuous readout is often essential and the type just proposed would be free of many of the limitations which characterize the standard dew point hygrometer.
Originally, as explained above, it appeared that by utilizing the unvarying physical phenomena centering on the hydration of a single "optically-responsive" salt the essential requirements could be met. A very small mass of a selected material could be illuminated and scanned by one of the widely available, exceptionally compact, "light source/ light responsive" pairs of optoelectronic devices to generate large electrical signals which could readily be used in indicating and controlling functions.
Unfortunately, the wide circulation of information regarding the availability of these patents for license (along with suggestions for the design and fabrication of what was termed a "saturated salt-type dew point" device) brought no successful development by any of a number of corporations which manufacture or use hygrometric devices. The reason was that the device which was so interesting theoretically could not be made to work in practice. Chemical compositions simply did not perform in the analogous device the way water (as dew) functions in the classic dew point hygrometer.
Subsequently, my extensive research and development has now established that what is required of a chemical composition (for it to perform through heating what water performs through cooling in a dew point device) includes the following novel features:
1) Long Term Stability
The sensing substance, water (as dew), is continually refurbished from the air in standard dew temperature devices. Thus, stability is of no concern. By way of comparison, the chemical composition of the proposed "saturated salt" device or, more accurately, "salt-solution transition temperature" device must be chemically and physically stable for long periods at high temperature in the presence of dust, etc. Such a device hereafter will be termed a "hot mirror" device, analogous to standard cold mirror devices.
2) Low Tendency to Supersaturate
Though liquid water as dew can be slightly supercooled before forming ice, a large number of salts of apparent use as humidity sensors are found in practice to supersaturate severely and with great ease. By "supersaturate" is meant to form solutions in which the solute is more highly concentrated than equilibrium curves would seem to permit. To function satisfactorily as a humidity sensor, the composition's tendency to supersaturate must be minimal or absent if serious error is to be prevented.
3) High Degree of Birefringence
The optical signals generated emanate from birefringent crystals. In order to have adequate sensitivity to changing humidity, the sensing layer of liquid and solid must be very thin. Thus, very small amounts of birefringent crystals must pass considerable amounts of light. This requires that the birefringence of the composition be very great, a phenomenon seldom encountered in nature.
4) High Melting Point
Hydratable inorganic compounds usually have high heat stability and high melting points, but few have high birefringence. Organic compounds theoretically have a better chance of exhibiting the essential high birefringence for optical signalling. However, they are ordinarily unstable thermally and/or melt at low temperatures. The humidity-sensing composition obviously cannot melt within the temperature range necessary for effective humidity sensing.
5) Rapid, Reversible Response to Shifting Equilibrium Points
Chemical compositions may react with water vapor at varying rates, depending on the "activation energy" of the particular compound. Most applications for direct readout, discrete sensor hygrometers (such as have been described above) do not require high speed hydration of the sensing compositions. In contrast, the sensing compositions suitable for hot mirror devices must have a high speed of hydration, born of low activation energies. Indeed, hydration must be almost instantaneous if severe "overshooting" and "hunting" of optical/electronic systems based on such sensing compositions is to be avoided.
The sensing composition also must preserve its integrity as a signalling agent at high temperatures by remaining reversible. That is, it cannot shift structure to a new crystal form which has wholly new features such as, for example, different water solubility. Neither can the chemical composition bind the water so tightly that it cannot readily release its water on a slight elevation of temperature.
6) High Water Solubility for Low Humidity Sensing
In order to effectively compete with standard dew temperature hygrometers, the hot mirror device should be capable of sensing rather low humidity levels (low dew point temperatures). This usually requires the use of salts having extremely high water solubilities (and thus low water vapor pressures).
There are exceptionally salts whose solubility is exceptionally high. In the patents mentioned above, the lowest relative humidity (RH) noted to cause a solid/solid transition is that for the rubidium salt of 3,3'4,4' bensophenone tetracarboxylic acid. The transition occurs at 18% RH at room temperature. This pure salt, however, is unusable in the device considered here because the extreme slowness with which it dehydrates makes it virtually an irreversible sensor. The lowest RH noted in the patents for a solid/liquid transition is that for the potassium salt of levulinic acid at 30% at room temperature. Thus, to sense low humidities is inherently very difficult.
To simultaneously meet all of the essential criteria mentioned is exceptionally complex and is what has accounted for the failure to devise workable hot mirror devices. These itemized requirements in several instances refer only to solid/liquid systems, though solid/solid systems are also possible. However, as noted before, there is a more limited choice of potential solid/solid sensors.